The long term objective of this project is the understanding of molecular evolution and population genetics of bacterial communities with the emphasis on genesis and intra- and interspecies spread of mobile genetic elements. Specifically, the genetic population structure of contemporary and "prehistoric" mercury resistant bacteria will be characterized and compared. The "prehistoric" bacteria will be isolated from permafrost grounds of Kolyma Lowland containing high titers of viable bacteria which were preserved in a frozen state for a time span from 5 hundred to 2.5 million years. The genetical population structure will be characterized by DNA sequencing of mercury resistance plasmids and transposons and the chromosomal rpoB gene. This will allow one to test directly the current calibrations of evolutionary molecular clock and determine the evolutionary time scale for the chromosomal allele drift in natural bacterial communities and for rearrangements and the spread of mobile DNA elements. The results will be important for the risk assessment of release of genetically engineered microorganisms into the environment and for the evaluation the long-term impact of industrial pollution on bacterial communities. Determination of currently unknown genetic population structure of soil bacterial species may have a specific medical significance, since strains belonging to some of these species are recently becoming the causes of serious hospital infections.